1. Field of the Invention
The above invention relates to a screen for the sifting of materials of differing weight.
The invention relates particularly to a screen for the grading of different materials which are contained, for example, in comminuted building rubble.
2. Description of Related Art
It is known to convey comminuted building rubble over screens or sifting devices, in order to obtain material accumulations with a defined maximum grain size. The material thus pregraded and sifted can be used as a substitute for natural aggregates, for example for making substrata, as a filler or for producing concrete mixtures of low strength.
The known sifters or plate-shaped vibrating screens with one or more sifting stations and with rotatably mounted sifters mostly consist of metal sheets or grids which are continuous or which are equipped with successive screening sections. These sections have sifting orifices of differing size. The larger orifices determine the maximum grain size both for the heavy rubble constituents (concrete particles, bricks, glass, tiles, pieces of marble) forming the actual natural aggregate substitutes and for the lighter materials (paper, cardboard, wood, plastic pieces) which constitute undesirable additives which reduce the quality of the recovered aggregates. To increase the quality of the recovered aggregates, it is necessary to reduce the fraction of light material as far as possible. This is possible, at the present time, only by manual sorting or by the use of special sifters which are operated by suction or flooding with water.
Manual sorting necessitates the constant presence of several assistants who perform their activity in an unhealthy environment with a high dust content. Sorting by suction effect requires the use of expensive devices, taking up a large amount of space, for generating the necessary vacuum and for separating the large quantities of dust sucked up. With such devices, moreover, the power consumption is very high. The discharge of the light materials as a result of their floatability in a water bath also makes it necessary to employ very bulky and expensive devices for the circulation of the water bath; a very large quantity of water, which is not always available, is needed. The separation of the lighter constituents in a water bath also leads to problems of environmental protection, and a high power consumption is also required in order to operate a water bath.
A further disadvantage of the known sifting devices is that it is not possible to make the screening orifices larger to any desired extent, as would be the case, for example, when heavy pieces of rubble of increasing grain size were to be sifted, without thereby having to allow for an undesirable increase of larger light constituents of the rubble in the screened-off fraction.